1. Field of the Invention
This invention relates to primary electrochemical cells utilizing an electrolyte comprising a solid polymer.
2. Description of the Prior Art
Aqueous electrolytes conventionally used as electrolytes in primary batteries, can be disadvantageous in that degradation of the electrodes can result by contact with said electrolytes. In addition aqueous electrolytes can be difficult to handle. Therefore solid electrolytes have been developed which have certain advantages including thermostability, absence of corrosion of the electrodes, and a wide range of redox stability which permits their combination with highly energizing couples to obtain electrochemical generators of high energy per unit of weight. Solid electrolytes are also advantageous because they can be prepared in thin layers which makes it possible to decrease the internal resistance of the electrochemical generator. Commercially available solid electrolyte battery systems utilize lithium iodide as the solid electrolyte since the lighter alkali metals, in particular lithium are the most attractive commercially utilized anode materials. Much research has been concentrated on lithium ion conductors as electrolytes but only little work has been done using alkaline earth metal ion conductors since the alkaline earth metal salts are poorly ionized. In a battery, the overall resistance of the fabricated electrolyte element limits the rate capability. In addition the volume taken up by a solid electrolyte such as lithium iodide is wasted space which could otherwise be devoted to active electrode components. Therefore, in order to maximize volumetric energy density and rate capability, the ability to fabricate a solid electrolyte as a thin element is important. Because, upon cell discharge, there is often a substantial redistribution of material in the cell and, in general, an overall volume change may occur, cell design must accommodate or minimize the stress on a thin, solid electrolyte element. In addition, the electrolyte must be compatible with the electrodes, both in the sense of being unreactive and also of making and maintaining electrical contact.
Solid polymer-salt complexes for use as electrolytes in electrochemical generators are known from U.S. Pat. No. 4,579,793, Armand et al in which there are disclosed electrolytes of cross linked organometallic polymers in which lithium salts are dissolved. The solid polymer-salt electrolytes of U.S. Pat. No. 4,578,326 to Armand et al are polyether copolymers which have been found to have improved conductivity over certain polyether homopolymers. Preferably sodium or lithium salts can be utilized as the ionic compound to be used in admixture with the polymer. The solid complexes of poly(ethylene oxide) and magnesium chloride which are disclosed by Yang et al in J. Electrochemical Society, July 1986, pp. 1380-1385 appear to be principally anion conductors when used as electrolytes. Cross linked solid polymers of a cyclic ether in admixture with alkali metal salts of weak bases are disclosed in U.S. Pat. No. 4,357,401 to Andre et al. In addition, solid polymer electrolytes containing a salt the anion of which is a residue derived from a strong acid and the cation of which is derived from an alkali metal or the ammonium ion are disclosed in U.S. Pat. No. 4,303,748 to Armand et al.
Novel alkali metal based ionic compounds are disclosed in U.S. Pat. No. 4,505,997 to Armand et al in admixture with solid polymers as electrolytes. The polymers are derived from monomer units which include at least one heteroatom, particularly oxygen or nitrogen, in the structure. Solid electrolytes are disclosed in U.S. Pat. No. 4,556,614 to Mehaute et al which include a first complexing polymer, an ionizable alkaline salt, and a second polymer having cross-linkable functions, for instance, a polymer of polyoxyethylene containing lithium perchlorate in admixture with a polymer of an acrylic modified polybutadiene-nitrile.
Complexes of lithium sodium or potassium salts and solid crown polyethers are disclosed as electrolytes in U.S. Pat. No. 3,704,174 to Berger. In U.S. Pat. No. 4,060,674 and U.S. Pat. No. 4,139,681 to Klemann electrolytes consisting of an organic solvent and an organometallic alkali metal salt are disclosed.
There is no indication in any of these references that useful electrolytes can be obtained from triphenylmethylhalo-borate, -arsenate, -antimonate, or -phosphate salts and solid polymers which provide anionic conductance at ambient temperature, i.e., 20.degree. to 100.degree. C.
It is an object of the invention to provide a solid electrolyte comprising a polymer-salt complex which provides anionic conductance. This is especially important where alkaline earth metal anodes are utilized in electrochemical cells in conjunction with the solid polymer-salt complex electrolytes since most salt solutions containing singly charged alkali metal ions are strongly ionized as compared with salt solutions containing doubly charged alkaline earth metal ions. The solid polymer-salt complex electrolytes are suitable for primary electrochemical cells operating at ambient temperatures such as 20.degree. to 100.degree. C. The electrolytes are particularly useful in combination with an alkaline earth metal anode such as an anode of magnesium or calcium. The polymer-salt complex electrolytes of the invention have good flexibility and provide high anionic conductivity when the ionic salt utilized in combination with the solid polymer is an organomethylhalo-borate, -arsenate, -antimonate, or -phosphate compound.
SUMMARY OF THE INVENTION
A polymer-salt complex electrolyte is disclosed which is useful in a primary electrochemical cell having an anode of an alkali or alkaline earth metal. The electrolyte is capable of providing anionic conductance and is particularly useful in electrochemical cells having an alkaline earth metal anode which operate at ambient temperatures of about 20.degree. C. to about 100.degree. C. The active cathode material of the cell is selected from at least one of the sulfides, halides, haloborates, haloarsenates and halophosphates of metals from groups Ib, IIb, IVa, Va, IVb, Vb, VIb, VIIb, and VIII of the Periodic Table of the Elements, quaternary tetraalkylammonium polyhalides, or an element selected from the group consisting of sulfur and iodine. The cathode can be, but need not be, a compound which intercalates the cation of which the anode is formed.
A useful method of forming the solid polymer-salt complex electrolyte is to form the polymer from a suitable monomer using a Lewis acid catalyst in the presence of an ionizing salt compound which is selected from at least one of a salt of the formula: EQU (R).sub.3 CMX.sub.n,
wherein M is selected from the group consisting of at least one of boron, phosphorus, antimony, and arsenic, and wherein X is halogen, n is 4 or 6, and R is aryl of 6-18 carbon atoms, alkyl of 1-8 carbon atoms, or alkaryl of 7-26 carbon atoms. R is preferably phenyl.
The polymer is formed from a monomer comprising at least one heteroatom in the monomer unit such as a heteroatom selected from at least one of the group consisting of oxygen, nitrogen, sulfur, and phosphorus. Alternatively, the polymer-salt complex can be formed by dissolving said salt in a preformed polymer either by means of a solvent or by fusion techniques, if the polymer is thermoplastic.